NASA

Employs about 18,000

17,450 staying home

Skeleton staff maintain key science and communications satellites and the two American astronauts on the International Space Station remain on duty.

Someone will be on hand to pick up the phone if they call Mission Control in Houston.

Work on new missions, including preparations to launch a Mars probe on November 18, have been suspended, NASA spokesman Allard Beutel said.

Agency websites are offline and have posted a message reading: "Due to the lapse in federal government funding, this website is not available."

NASA newsrooms across the country are closed.

Department of Education

Employs 4,225

4,013 staying home

Funding due to be handed out to schools will still be distributed.

On its website, the department has posted a message which reads: "Due to a lack of appropriations, ED activities have been curtailed and most employees are on furlough. ED.gov will not be updated during the shutdown."

National Parks

Employs 24,000

21,000 staying home

America's 401 national parks, monuments, historic houses and battlefields are now closed.

Overnight visitors to national parks have been given two days to depart.

This will mean a loss of 750,000 daily visitors and an economic loss to gateway communities of as much as $US30 million for each day parks are shut, according to the non-profit National Parks Conservation Association.

Department of Agriculture

Meat inspectors are staying on the job. Agricultural statistical reports have ceased publication, and the important October 11 crop report could be delayed depending on how long the shutdown lasts.

USDA's main website, USDA.gov, has gone dark and has been linked to a page explaining the shutdown.

The White House

Employs 1,700

1,265 staying home

Among the staff retained will be 15 to provide "minimum maintenance and support" for the White House.